Seong‐Jae Doh

We use bivariate scatter plots to illustrate variations in selected rock magnetic properties (low‐field susceptibility, anhysteretic and isothermal remanence) of late Neogene and Pleistocene deep‐sea sediments from 16 sites in the Arctic Ocean, North Atlantic, equatorial Atlantic and North Pacific Ocean, and the Arabian Sea. Our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology, geographical area, and the dominant mode of terrigenous sedimentation, which at these sites is via ice‐rafting, via bottom currents, or via eolian processes. Overall, correlations between sediment magnetic properties and gross lithology is poor, and factors such as the source and transport path of terrigenous sediment (and detrital magnetic minerals), together with the action of reductive diagenetic processes, are the major controls on the magnetic properties. On the bivariate scatter plots, sites with major ice‐rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations, relatively coarse ferrimagnetic grain‐sizes, and scattered sample point distributions; in contrast, sites where we infer significant bottom‐current supply of terrigenous material have tightly grouped sample point distributions. Carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration, caused by glacial‐interglacial fluctuations in carbonate accumulation and eolian activity. Sediments containing significant volcanogenic material have high concentrations of relatively coarse‐grained ferrimagnetic material. Reductive diagenesis is a significant determinant of sediment magnetic properties in high‐productivity areas and has the effect of preferentially removing the fine‐grained ferrimagnetic fraction, causing a coarsening of the ferrimagnetic grain‐size distribution and a rise in the antiferromagnetic:ferrimagnetic ratio.

Previous magnetostratigraphic studies of nonfossiliferous pre‐Pliocene age pelagic clay cores from the central north Pacific have proven unsuccessful because of the unstable behavior of the natural remanent magnetization. The inability to obtain a reliable magnetostratigraphy has severely limited the temporal resolution that can be achieved in paleoceanographic studies of these nonfossiliferous pelagic clays. We carried out a rock‐magnetic study of core LL44‐GPC3, which spans the interval 0‐70 m.y. ago, to determine whether rock‐magnetic parameters can be used to construct a high‐resolution stratigraphic framework for paleoceanographic studies of central north Pacific pelagic clays despite the instability of the natural remanence. In addition, we tested the ability of rock‐magnetic methods to detect and characterize the paleoceanographic changes that are recorded in the sediments of LL44‐GPC3. Stratigraphic variations in rapid and nondestructive rock‐magnetic parameters and related ratios reflect changes in the concentration, mineralogy, and grain size of the magnetic minerals within the sediments. Rock‐magnetic parameters exhibit coherent fluctuations within both the stable and unstable sections of LL44‐GPC3. This result suggests the potential use of these parameters for regional correlation and relative dating of the nonfossil‐bearing pelagic clays of the central north Pacific gyre. Major fluctuations in rock‐magnetic parameters plotted as accumulation rates correspond to intervals of paleoenvironmental change that have been previously detected by other paleoceanographic methods. In addition, variations in a parameter proportional to the concentration of goethite/hematite correlate remarkably well with the mass accumulation rate of the total eolian component, suggesting that in some situations it can be used as a proxy indicator for eolian activity.